The present invention relates to the removal of NO.sub.x from gases such as flue gases. The gas containing the oxides of nitrogen is contacted with a culture of facultatively anaerobic bacteria capable of using nitrate as a terminal electron acceptor to effect the chemical reduction to elemental nitrogen.
A need exists for new technology for the disposal of concentrated gas streams containing oxides of nitrogen, NO.sub.x (NO and NO.sub.2), as obtained from certain regenerable, dry scrubbing processes for flue gas desulfurization, such as the NOXSO process (Yeh et al, "The NOXSO Process: Simultaneous Removal of SO.sub.2 and NO.sub.x from Flue Gas", paper presented at the AICHE Spring National Meeting, Houston, Tex. (March, 1987)), and the removal and disposal of NO.sub.x from more dilute gas streams such as produced by nitric acid plants.
Combustion in air inevitably produces oxides of nitrogen due to the reaction at high temperatures of elemental nitrogen or fuel nitrogen with oxygen. Roughly 90-95% of the oxides of nitrogen emitted in combustion processes is in the form of nitric oxide (NO). The remainder is predominately nitrogen dioxide (NO.sub.2). In the atmosphere NO is converted in time to NO.sub.2. Since NO and NO.sub.2 generally coexist in varying proportions in flue gases and in the atmosphere, they are frequently lumped together under the generic formula NO.sub.x.
In urban metropolitan areas where NO.sub.x emission sources are concentrated, NO.sub.x reacts with hydrocarbons in the atmosphere in photochemical reactions to produce smog. The chemical components of smog, particularly organic peroxy nitrates and ozone have a direct adverse effect on human health and plant life.
NO.sub.x emissions may be controlled in basically two ways. First, emissions may be reduced by decreasing the residence time of combustion gases at peak flame temperatures (high temperature favors NO.sub.x formation) and reducing the availability of oxygen. However, these measures can be expensive to implement and, in the latter case, result in increased emissions of carbon monoxide, another serious pollutant. Where combustion control is not feasible, NO.sub.x must be removed from the cooled flue gases before they are released into the atmosphere. However, flue gas cleaning for NO.sub.x removal has been severely limited by the low reactivity of nitrogen oxides and the large volume of gas to be treated at most stationary combustion sources.
Thiobacillus denitrificans is a strict autotroph and facultative anaerobe first described in detail by Baalsrud & Baalsrud (Arch. Microbiol., 20, 34 (1954)). Under anaerobic conditions, nitrate may be used as a terminal electron acceptor with reduction to elemental nitrogen. Thiosulfate, elemental sulfur and sulfide may be used as energy sources with oxidation to sulfate. Nitric oxide (NO) has been shown to be an intermediate in the reduction of nitrate to elemental nitrogen in T. denitrificans. Ishaque & Aleem (Arch. Mikro, 94, 269 (1973)) and Baldensparger & Garcia (Arch. Mikro., 103, 31 (1975)) have demonstrated that whole cells of T. denitrificans will catalyze the reduction of nitric oxide to elemental nitrogen with a concomitant oxidation of thiosulfate (electron donor). However, these experiments utilized "resting cells"; that is, the cells were not actively growing and reproducing. Since nitric oxide reduction in T. denitrificans would be directly linked to the energy metabolism of the cell, the highest specific activity for NO reduction should occur when cells are actively growing and reproducing.